1. Field of the Invention
The present invention relates to an industrial robot, and more specifically to a multijoint link industrial robot improved in both operating space and load capacity.
2. Description of the Prior Art
The multijoint industrial robots can be classified into horizontal multijoint robots provided with joint members movable mainly in a horizontal plane and vertical multijoint robots provided with joint members movable in a vertical plane.
In the case of the vertical multijoint robots, there are some whose operating space is limited due to interference of driven links with each other. In the case of a robot as shown in FIG. 1, however, since an upper arm member 21 having a wrist member 20 at an end thereof can be pivoted sufficiently upward and rearward, it is possible to increase the operating space. In other words, in the vertical multijoint robot as shown in FIG. 1, since the supporting member and the supported member are not interfered with each other in each of the pivotably constructed joint portions J.sub.a and J.sub.b (between a base member 24 and a rotary trunk member 23 or between a lower arm member 22 and an upper arm member 21), it is possible to increase the operating space.
In the vertical multijoint robot as shown in FIG. 1, however, when the lower arm 22 is pivoted, the upper arm 21 is pivoted together with the lower arm 22 and therefore, the load torque of a driving motor for pivoting the lower arm 22 greatly inevitably increases. In the case of a heavy load robot (which implies a robot which can handle heavy goods), in particular, when the lower arm 22 is pivoted to nearly a horizontal position, the load torque of the driving motor 25 for pivoting the lower arm 22 increases greatly. In more detail, with reference to a model diagram shown in 2, when a load of W kgf is applied to an end of the upper arm 21 (the arm length is l.sub.2 and the pivotal angle is .theta..sub.2), the moment M generated at the pivotal axis of the lower arm 22 (the arm length is l.sub.1 and the pivotal angle is .theta..sub.1) can be expressed as EQU M=W.times.(l.sub.1 sin.theta..sub.1 +l.sub.2 sin.theta..sub.2)=W.times.l
Therefore, the load torque increases greatly in proportion to the horizontal distance l between the pivotal axis and the load (W) action line. Accordingly, it is practically impossible to adopt such a structure as described above to the heavy load robot.
On the other hand, in the case of the vertical multijoint robots, there are two types of direct acting robot (in which each arm is directly driven by each driving motor to increase the operating space) and a parallel link robot (in which the arm is driven by a parallel link mechanism from the arm side of the base to reduce the drive torque).
In a conventional parallel link robot, however, there exist problems in that the links interfere with each other and further a dead point is easily produced in the parallel links, so that it has been difficult to increase the operating angle. To overcome these problems, Japanese Laid-open Patent (Kokai) No. 62-228385 (1987) discloses a robot as shown in FIGS. 3A and 3B, for instance, in which an upper arm 26 is pivotably supported by a lower arm 27; a driven rotary plate 28 is rotatably supported at the pivotal axis O.sub.1 of the upper and lower arms 26 and 27; another driving rotary plate 29 is rotatably supported at the pivotal axis O.sub.2 of the lower arm 27 and the rotary trunk 32 for supporting the lower arm 27; the two rotary plates 28 and 29 are connected via two links 30 and 31; and the upper arm 26 is moved or oscillated about the pivotal axis O.sub.1 via the two links 30 and 31 when the driven rotary plate 29 is driven by a driving motor 33. Further, the lower arm 27 is driven by another driving motor 34 so as to be pivoted or inclined about the pivotal axis O.sub.2.
In the case of the parallel link robot, however, there exist problems in that three links including the lower arm 27 are required and therefore the structure is complicated. In addition, since these links interfere with each other, the rearward movement of the arms is inevitably restricted. That is, when two rotary plates 28 and 29 rotate by a predetermined angle, the two links 30 and 31 are interfered with each other, with the result that the motion of the upper arm 26 is restricted. In practice, the upper arm 26 cannot pivot perfectly rearward. Therefore, when the operating space is changed from the front space to the rear space, the rotary trunk 32 must be pivoted relative to a base member (not shown). However, when the rotary trunk member 32 is rotated for rear side work as described above, there arises another problem in that a broad arm pivoting space is required around a single robot, because the lower arm 27, the upper arm 26 and the links 30 and 31 extending from the pivotal axis O.sub.3 move within each operating space determined by each large locus or each large radius, respectively. In other words, when a great number of robots are arranged along an automotive vehicle assembly line, for instance, it is necessary to have a large space to install each robot.